Acoustoelectronic Structures with an Air Gap for Gas Analysis

A contact less surface acoustic wave (SAW) device based on the acoustoelectronic structure with an air gap (ASWAG) and its application as gas sensor is presented. ASWAG sensor utilizes layered structure based on piezoelectric substrate (LiNbO₃, air gap (0.2 m) and semiconductor (Si), in which the semiconductor plate is remotely excited by SAW accompanying RF waves for generation transverse acoustoelectric effect (TAE). The concept of the ASWAG device along with the essential theoretical underpinnings of the functioning of TAE based sensors has been reported. In particular, the general theoretical analysis of the acoustoelectric effect as a function of various electro induced processes on the semiconductor surface under gas adsorption has been provided. In order to show the suitability of this type of device for sensor applications, we present several specific examples of how 37 MHz ASWAG devices for gaseous analysis and materials investigations, in particular, for characterization the semiconductor surfaces and gas-phase species detection such as ethyl alcohol, water and water solutions of acetic acid and ammonia have been used. A critical discussion is given of several approaches to chemical selectivity, interaction mechanisms and the many practical issues of importance for environmental sensing. Anticipated future trends and prospects for tuneable and multi-channel measurements are also presented.